U.S. Pat. No. 4,111,938 is directed to the preparation of 2,3,5-trichloropyridine, an intermediate for the production of pesticides. The trichloropyridine is obtained by contacting a solution of pentachloropyridine (PCP) or 2,3,5,6-tetrachloropyridine (Sym-tet) in a water-immiscible solvent with zinc dust in the presence of an alkaline reagent having a pH of at least 11. The latter reagent preferably is aqueous NaOH.
Solvents named in the patent as being representative are benzene, toluene, xylene, ethyl benzene, hexane, heptane and anisole. The best yield (77% of theoretical) of the trichloro compound disclosed was obtained by refluxing at .about.79.degree. C. for five hours a mixture of 4 gram atoms of zinc dust and 1.25 liters of 8N aq. NaOH with a solution of 1 g. mole of PCP in 500 ml. of benzene. Substantially lower yields were obtained when the solvent was toluene or isopropanol. When 2-methoxy1-propanol was employed as the solvent, the yield of the trichloro compound was only 12% of theoretical.
No indication is given in the patent as to the relative rates of reduction of PCP and Sym-tet and it cannot be ascertained whether the disclosed method would be practical for the selective manufacture of 2,3,5,6-tetrachloropyridine.
Since the '938 patent issued (1978), Sym-tet has become an increasingly important intermediate for the production of the commercial insecticides DURSBAN* and LORSBAN* and the commercial herbicide GARLON* (*trademarks registered to The Dow Chemical Company). A variety of methods of preparing this intermediate have been devised, including the use of zinc dust in neutral or acidic media to reduce PCP dissolved in solvents such as acetic acid, isobutanol or dimethyl methylphosphonate. Two of these methods have been reported to give Sym-tet yields of up to about 91% of theoretical, but all of them leave something to be desired in one respect of another.